This invention relates to high temperature superconductor (HTS) mini-filters and mini-multiplexers with self-resonant spiral resonators as the building blocks, which have the advantages of very small size and very low cross-talk between adjacent filters.
HTS filters have the advantages of extremely low in-band insertion loss, high off-band rejection, steep skirts, due to extremely low loss in the HTS materials. The HTS filters have many applications in telecommunication, instrumentation and military equipment. However, for the regular design of a HTS filter, the resonators as its building blocks are large in size. In fact, at least one dimension of the resonator is equal to approximately a half wavelength. For low frequency HTS filters with many poles, the regular design requires a very large substrate area. The substrates of thin film HTS circuits are special single crystal dielectric materials with high cost. Moreover, the HTS thin film coated substrates are even more costly. Therefore, for saving material cost, it is desirable to reduce the HTS filter size without sacrificing its performance. Furthermore, for the HTS filter circuits, the cooling power, the cooling time, and the cost to cool it down to operating cryogenic temperature increases with increasing circuits' size. These are the reasons to reduce the HTS filter size without sacrificing its performance.
There is a prior art design to reduce the HTS filters size, i. e. by using lumped circuit" elements such as capacitors and inductors to build the resonator used as the building blocks of HTS filters. This approach does reduce the size of HTS filters. However, it also has problems. First, the regular element inductors such as the spiral inductors shown in FIGS. 1a and 1b have wide spread magnetic fields, which reach the region far beyond the inductor and undesirable cross-talk between adjacent circuits. Second, in the lumped circuit filter design, the two ends of the spiral inductor must be connected to other circuit components such as capacitors etc. But one of the inductor's two ends is located at the center of the spiral, which cannot be directly connected to other components. In order to make the connection from the center end of the spiral inductor to another component, an air-bridge or multi-layer over-pass must be fabricated on top of the HTS spiral inductor. They not only degrade the performance of the filter, but also are difficult to fabricate. Third, there are two ways to introduce lumped capacitors: One is using a "drop-in" capacitor, which usually has unacceptable very large tolerance. The other is using a planar interdigital capacitor, which requires a very narrow gap between two electrodes with high rf voltage across them, which may cause arcing.
The purpose of this invention is to use self-resonant spiral resonators to reduce the size of HTS filters and at the same time to solve the cross-talk and connection problems.